sed substitution in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of sed substitution with global flag

What is the output of this command when run on the input string apple apple apple?

Linux CLI

echo "apple apple apple" | sed 's/apple/orange/g'

A

orange apple apple

B

orange orange orange

C

apple orange apple

D

apple apple orange

💡 Hint

The g flag means replace all occurrences in each line.

The s/apple/orange/g command replaces every 'apple' with 'orange' in the input line.

💻 Command Output

intermediate

Effect of sed substitution without global flag

What is the output of this command when run on the input string banana banana banana?

Linux CLI

echo "banana banana banana" | sed 's/banana/grape/'

A

banana banana grape

B

grape grape grape

C

grape banana banana

D

banana grape banana

💡 Hint

Without the g flag, only the first match is replaced.

The s/banana/grape/ command replaces only the first 'banana' in the line.

📝 Syntax

advanced

Identify the correct sed substitution syntax

Which option shows the correct syntax to replace all occurrences of cat with dog in a file named pets.txt?

A sed 's/cat/dog/g' pets.txt

B sed s/cat/dog/g pets.txt

C sed "s/cat/dog/g" pets.txt

D sed -s 'cat' 'dog' pets.txt

💡 Hint

The substitution command should be enclosed in single quotes with the s///g format.

Option A uses the correct syntax with single quotes and the substitution command.

💻 Command Output

advanced

Output of sed substitution with special characters

What is the output of this command?

Linux CLI

echo "path/to/file" | sed 's/\//-/g'

A

path/to/file

B

path-to/file

C

path\to\file

D

path-to-file

💡 Hint

The slash character is escaped to be replaced by a dash globally.

The command replaces all slashes '/' with dashes '-' in the input string.

💻 Command Output

expert

Result of sed substitution with capture groups and backreferences

What is the output of this command?

Linux CLI

echo "2024-06-15" | sed 's/\([0-9]\{4\}\)-\([0-9]\{2\}\)-\([0-9]\{2\}\)/\3\/\2\/\1/'

A

15/06/2024

B

06/15/2024

C

2024/06/15

D

15-06-2024

💡 Hint

Backreferences \1, \2, \3 refer to year, month, day respectively.

The command rearranges the date from YYYY-MM-DD to DD/MM/YYYY format using capture groups.

Practice

1. What does the sed command sed 's/cat/dog/' file.txt do?

easy

A. Replaces the first occurrence of 'cat' with 'dog' on each line of file.txt

B. Replaces all occurrences of 'cat' with 'dog' in the entire file.txt

C. Deletes all lines containing 'cat' in file.txt

D. Prints lines containing 'dog' from file.txt

Solution

1. Step 1: Understand the sed substitution syntax

   The command sed 's/cat/dog/' file.txt uses s/pattern/replacement/ which replaces the first match per line.

2. Step 2: Apply the command to the file

   It replaces only the first 'cat' found on each line with 'dog', not all occurrences or lines.

3. Final Answer:

   Replaces the first occurrence of 'cat' with 'dog' on each line of file.txt -> Option A

4. Quick Check:

   sed s/pattern/replacement/ replaces first match per line [OK]

Hint: No 'g' flag means only first match per line replaced [OK]

Common Mistakes:

• Thinking all matches are replaced without 'g'
• Confusing substitution with deletion
• Assuming it prints only matching lines

2. Which of the following is the correct syntax to replace all occurrences of 'apple' with 'orange' using sed?

easy

A. sed 's/apple/orange/' file.txt

B. sed 's/apple/orange/g' file.txt

C. sed 's/apple/orange/g;' file.txt

D. sed 's/apple/orange/gp' file.txt

Solution

1. Step 1: Identify the flag for global replacement

   The 'g' flag after the substitution pattern tells sed to replace all occurrences on each line.

2. Step 2: Check syntax correctness

   sed 's/apple/orange/g' file.txt uses sed 's/apple/orange/g' which is the correct syntax without extra characters.

3. Final Answer:

   sed 's/apple/orange/g' file.txt -> Option B

4. Quick Check:

   'g' flag means replace all matches per line [OK]

Hint: Add 'g' after last slash to replace all matches [OK]

Common Mistakes:

• Omitting the 'g' flag for all replacements
• Adding extra semicolons inside quotes
• Using 'gp' which is invalid here

3. What is the output of this command if input.txt contains:

apple apple banana
banana apple apple

sed 's/apple/orange/' input.txt?

medium

A. orange orange banana
banana orange orange

B. orange apple banana
banana apple apple

C. orange apple banana
banana orange apple

D. apple apple banana
banana apple apple

Solution

1. Step 1: Understand sed substitution without 'g'

   Without 'g', only the first 'apple' on each line is replaced by 'orange'.

2. Step 2: Apply substitution line by line

   Line 1: 'apple apple banana' becomes 'orange apple banana'. Line 2: 'banana apple apple' becomes 'banana orange apple'.

3. Final Answer:

   orange apple banana banana orange apple -> Option C

4. Quick Check:

   First match per line replaced only [OK]

Hint: No 'g' means only first match per line changes [OK]

Common Mistakes:

• Replacing all 'apple' without 'g'
• Replacing none due to misunderstanding
• Confusing line order or content

4. Identify the error in this sed command: sed 's/hello/world/g file.txt'

medium

A. File name should come before the command

B. Incorrect delimiter used

C. Missing 'g' flag for global replacement

D. Missing closing quote after 'g'

Solution

1. Step 1: Check the quotes in the command

   The command has an opening single quote before s/hello/world/g but no closing quote after 'g'.

2. Step 2: Confirm correct placement of quotes and arguments

   The file name should be outside the quotes. The missing closing quote causes syntax error.

3. Final Answer:

   Missing closing quote after 'g' -> Option D

4. Quick Check:

   Quotes must enclose entire sed expression [OK]

Hint: Always close quotes around sed expression [OK]

Common Mistakes:

• Placing file name inside quotes
• Forgetting closing quote
• Confusing delimiter with quotes

5. You want to replace all occurrences of 'foo' with 'bar' only on lines containing 'baz' in data.txt. Which command achieves this?

hard

A. sed '/baz/ s/foo/bar/g' data.txt

B. sed 's/foo/bar/g /baz/' data.txt

C. sed 's/foo/bar/g' data.txt | grep baz

D. sed '/foo/ s/baz/bar/g' data.txt

Solution

1. Step 1: Understand address restriction in sed

   Using /baz/ s/foo/bar/g applies substitution only on lines matching 'baz'.

2. Step 2: Verify command correctness

   sed '/baz/ s/foo/bar/g' data.txt correctly restricts substitution to lines with 'baz' and replaces all 'foo' with 'bar'.

3. Final Answer:

   sed '/baz/ s/foo/bar/g' data.txt -> Option A

4. Quick Check:

   Address before command limits lines affected [OK]

Hint: Use /pattern/ before s/// to limit lines [OK]

Common Mistakes:

• Placing substitution after file name
• Using grep after sed instead of address
• Mixing patterns in wrong order